The present invention relates to circuitry for affecting the characteristics of a telephone transmission line to thereby improve signal transmission through the line and more particularly to circuitry which, by the generation of a single voltage and a single current, causes a multiplicity of simultaneous non-interactive effects on the transmission characteristics of the line.
In communication systems wherein voice signals are transmitted over substantial distances through transmission lines, it is necessary to provide circuitry which can compensate for the attenuation of the signals by the transmission line. In telephone systems, for example, it is necessary to provide amplifier circuits or repeaters to maintain satisfactory signal transmission through telephone lines which, in the absence of such circuits, would excessively attenuate the signals transmitted therethrough.
In the development of circuitry for transmitting voice frequency signals over transmission lines, a variety of types of repeater circuits have been employed. One of these types of repeater circuits is a repeater having a series amplifying network for inserting in series with the transmission line, an amplifying voltage which varies in accordance with the siganl voltage across the transmission line and a shunt amplifying network for inserting, in shunt with the transmission line, an amplifying current which varies in accordance with the signal current through the transmission line. In such circuits, it may be shown that if the ratio of amplifying voltage to signal voltage is equal in magnitude, but opposite in sign to the ratio of amplifying current to signal current, the circuit may function as an impedance matching circuit. It may also be shown that if the above ratios are equal in magnitude and have the same sign, then the circuit may function as a repeater which compensates for the frequency independent attenuation of loaded transmission lines or the frequency dependent attenuation of non-loaded transmission lines. One such repeater which compensates for the attenuation of signals in a loaded transmission line is shown and described in U.S. Pat. No. 3,706,862 granted in the name of C. W. Chambers, Jr. on Dec. 19, 1972. A repeater which compensates for the attenuation of signals in a non-loaded transmission line is shown and described in U.S. Pat. No. 3,818,151 granted in the name of C. W. Chambers, Jr. et al on June 18, 1974. Repeater circuits of these types are referred to as amplifying type repeaters.
Another of these types of repeater circuits is a repeater having impedance simulating networks which provide gain by simulating the presence of negative resistances (or impedances) in series and/or in shunt with the transmission lines. These impedance simulating networks may also be utilized to simulate the presence of positive impedance and thereby serve as line-build-out networks or attenuator pads. One such type of repeater is shown and described in U.S. Pat. No. 3,828,281 granted in the name of C. W. Chambers, Jr. on Aug. 6, 1974. Circuits of either of these types are referred to as impedance simulating type networks.
The function performed by each of the types of repeater circuits described above can be referred to as line conditioning. Hereinafter these types of repeater circuits will each or both or in any combination be referred to as line conditioning units (LCU's). It should be understood that a line conditioning unit typically comprises an input voltage signal processing means for sensing the signal voltage across the transmission line, an input current signal processing means for sensing the signal current through the transmission line, an output voltage signal processing means for generating a voltage for insertion in series with the transmission line, an output current signal processing means for generating a current for insertion in shunt with the transmission line and one or more networks coupled between the input voltage and current processing means and the output voltage and current processing means.
In particular, an amplifying type LCU has a first "amplifying" network coupled between the input voltage processing means and the output voltage processing means and a second "amplifying" network coupled between the input current processing means the the output current processing means. Hereinafter these "amplifying" networks will be referred to as gain control means. An impedance simulating type LCU has a first "impedance" network coupled between the input voltage processing means and the output current processing means and a second "impedance" network coupled between the input current processing means and the output voltage processing means. Hereinafter these "impedance" networks will be referred to as impedance simulating control means.
Due to the relatively high cost of purchasing and operating a separate repeater for each transmission line, it has been found desirable to operate line conditioning units in a common mode configuration i.e., a configuration in which a relatively small number of units is switched among a relatively large number of occasionally used transmission lines. For amplifying type repeaters, circuitry can be provided which automatically varies the magnitude of the amplifying voltages and currents which are applied to a transmission line in accordance with the a-c losses of that line so as to establish the same system losses for transmission lines of differing lengths and gauges. For impedance simulating type circuits, circuitry can be provided which automatically varies the magnitude of the impedance simulating voltages and currents which are applied to a transmission line in accordance with the a-c losses of that line. For transmission lines of differing lengths and gauges, such circuitry is shown and described in U.S. Pat. No. 3,989,906 entitled "Repeater for Transmission Lines" which issued on Nov. 2, 1976 in the name of Frederick J. Kiko and also in U.S. Pat. No. 3,989,907 entitled "Repeater for Transmission Lines of Differing Lengths" which also issued on Nov. 2, 1976 in the name of Charles W. Chambers, Jr.
In telephone systems, it is often necessary to group combinations of line conditioning units so as to provide multiple functions. When, for example, an amplifier is to be located at a point along the length of a transmission line, it is often found that the impedance looking into the transmission line in one direction is substantially different from the impedance looking into the transmission line in the opposite direction. In the presence of an amplifying type repeater, this mismatch in line impedances can give rise to signal reflection and to less than complete transmission of signal power through the amplifier. Under these circumstances, the amplifiers are usually coupled to the transmission line through a pair of line build-out networks which match the impedances of the line looking towards each party. Prior to the present invention, the combination of line conditioning units which would provide both amplification and the desired impedance match required that these networks be coupled to the line through separate transformers so as to avoid interaction between the functions performed by each of the networks. When so connected, each line conditioning unit introduces into the transmission line a voltage and current which provides the desired effect.
It was then recognized that it would be far more desirable to introduce into the transmission line a single voltage in series with the line and a single current in shunt with the line which voltage and current affect the transmission characteristics of the line in a manner identical to the manner in which the characteristics of the line are affected by the introduction into the line of a multiplicity of series voltages and shunt currents, however, no technique or circuit was known which would produce these results.
In accordance with the present invention, there is provided circuitry which allows the insertion into a transmission line of a single voltage and a single current which voltage and current affect the transmission characteristics of the line in a multiplicity of independent non-interactive respects simultaneously. In addition thereto, such voltages and/or currents may also vary as a function of the a-c losses of the transmission line so as to allow lines of differing gauges and lengths to be grouped together in a common mode configuration.